June 19, 2003 HOUSTON, June 18, 2003 – On warm summer nights, biologists armed with nothing more than flashlights listen, crawl and search for one University of Houston campus resident that may, one day, yield new knowledge about evolution and the environment.
Dan Wells, professor of biology and biochemistry at UH, and his team are rummaging the campus undergrowth this summer in search of the Rio Grande chirping frog. The tiny amphibian is the only direct developing frog, meaning it hatches directly to a frog from an egg, outside a tropical climate.
Wells hopes to find out how and why the frog has developed the ability to skip a lifecycle vital to other amphibians. "Amphibians are supposed to lay their eggs in water, hatch, swim around and metamorphose to become terrestrial – to a large extent, that defines an amphibian," Wells said. "This frog doesn’t like water at all. It buries its eggs in the ground and, in about three weeks, they hatch out as tiny froglets."
Wells believes the source of this unusual lifecycle is a genetic master switch, or a single gene that controls metamorphosis. "It’s a quantum leap for our understanding," he said. "Master genes are not a completely far fetched notion – they have been found. For example, master genes control sexual development in mammals – if the gene is on, you form male structures, if it is off, you form female structures." A key element to a genetic master switch is a protein called a thyroxin receptor, which determines what organs the frog develops at what time. For instance, the Mexican axolotl, an amphibian with a thyroxin deficiency, is water-bound its entire life. The Rio Grande chirping frog may also be used as a guide to the quality of the local environment.
"Amphibians in general can be used as good monitors of environmental pollution," Wells said. "By studying defects, you can monitor what pesticides, pollution or ultraviolet light affect frog embryos." Before major questions can be answered, Wells said a major stumbling block must first be overcome.
"To know the evolutionary and environmental answers, we need to understand early development, such as how the early embryo forms," he said. "To be able to do that, we need to be able to raise the frogs." Collecting eggs and embryos from the wild has proved all but impossible. Last year, the team found no eggs at all.
The frogs have been successfully kept in captivity in a UH lab, but breeding the frogs has proved elusive. So, the Wells team decided to go back to basics – understand the frogs in their environment, including what they eat and how they survive day-to-day.
Wendy Orth, a biology graduate student, has collected the frogs to find their preferred habitat and diet and is recording the characteristic chirping calls. While searching for basic information, Orth may have stumbled onto something larger – the frogs may have evolved away from their original populations.
The frog’s only natural habitat is in the southern tip of Texas. It was introduced to Houston and San Antonio by way of the potted plant; 1984 was the first confirmed sighting. It is now spread over much of southwest Texas.
"I have noticed some slight differences in calls," Orth said. "The calls are important because that’s how they mate – a frog from Houston may no longer be able to mate with a frog from the original population in the southern tip of Texas."
If the frog chirps have changed, it may signify that the Houston population of frogs has evolved to a unique and distinct version of the Rio Grande chirping frog.
"If a population is isolated, it can develop its own characteristics so much that it can become its own species," Orth said. Once the breeding program is in place, the biologists can measure how much the populations have diverged from one another. Until then, Orth’s work will continue.
"On warm summer evenings, you’ll see us crawling around the bushes with flashlights and headlamps, listening for a call and then searching," she said. "The frogs are everywhere, you can hear them chirping all around you. But it’s a slow process. On a good night, five or six people may only catch 10 frogs."
Sidebar – Rio Grande Chirping Frog Facts History and Unusual Traits
The Rio Grande chirping frog is an amphibian native to Cameron and Hidalgo counties in the southern tip of Texas and northern Mexico. It has since migrated to Houston via the potted plant and is now firmly established in the Bayou City and the University of Houston campus. The chirping frog has evolved to skip the tadpole stage of a normal amphibian’s life, a feat that other frogs have achieved, but only in tropical environments.
Diet and Chirping
This tiny amphibian does not have a favorite food. It will eat spiders, flies, centipedes, termites – anything smaller than about two millimeters in length. The frog makes a distinctive call, or chirp, that many Houstonians mistake for crickets or other insects, and can be heard clearly on warm summer evenings.
Its adoptive, and favorite, habitat is a well-watered garden or potted plant. The frogs are particularly fond of UH’s myriad bushes and gardens – so much so, in fact, that biologists at the University of Houston studying the unusual amphibian need not travel any further than outside their office.
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